screen – Holding down the option key isn’t allowing me to see boot menu on Imac mid-2010

I get an old iMac mid 2010 without hard drive and when I press “alt” on windows keyboard ( equivalent of “option” on Mac ) nothing appens . Then appears the Apple logo with a charge bar and at the 3/4 of it the screen turn black .I try to reset nvram and change USB key but I can’t see the menu!!
Please help me